Abstract Amyotrophic lateral sclerosis (ALS) is a progressive degenerative disease that affects motor neurons. Mutations in the gene SOD1 (superoxide dismutase 1) and in chromosome 9 seem the most prevalent in those affected by the disease. Despite tremendous efforts aimed at identifying contributing factors for ALS, the mechanisms underlying motor neuron death have not yet been fully elucidated and consequently no effective treatment is currently available for ALS. Several clinical trials have been initiated based on drugs selected from animal studies, however, these ultimately failed. Obviously among the possible reasons for such failures, is the lack of a proper drug target responsible for the onset and progression of ALS. In this regard, numerous recent studies clearly suggest the EphA4 is a potential drug target for ALS and that targeting its ligand?binding domain may provide a possible avenue to novel and effective therapeutics. Based on these premises, we have recently designed and synthesized a novel EphA4 binding agent, named 123C4, targeting its ligand binding domain 123C4 exhibits nanomolar affinity for the EphA4 receptor with > 10 fold selectivity over the closest receptor of the family (EphA3), are brain penetrant and show notable efficacy in a SOD1 mutant mouse model of ALS. Our studies aimed at further optimizing and characterizing this series will provide critical insights on the role of the EphA4 modulation in the progression of ALS, and the data gathered in this study will be critical in supporting the development of these agents into innovative targeted therapeutics for ALS.